When a core is used in an alternating magnetic field, a loss of energy called iron loss occurs. This iron loss is expressed by the sum of hysteresis loss and eddy-current loss. To reduce hysteresis loss, the coercive force Hc of the core may be reduced. To reduce eddy-current loss, the electrical resistivity ρ of the core may be increased. In particular, in the use of the core at high frequency, eddy-current loss is significantly increased.
Dust cores disclosed in PTLs 1 and 2 are known as dust cores that can reduce the iron loss. The dust cores are formed by compacting composite magnetic particles that are obtained by forming an insulation coating on a surface of each of soft magnetic particles. Since the soft magnetic particles are insulated from each other by the insulation coating, the dust cores produce a high effect of reducing eddy-current loss.
Such a dust core is produced through a forming step of obtaining a compact using a mold including a die and a punch and a heat-treating step of performing a heat treatment on the compact to obtain a heat-treated compact. However, the shape of the compact obtained using the mold is limited to somewhat a simple shape, and furthermore it is difficult to stably maintain high dimensional accuracy. Therefore, the shape of a dust core obtained is sometimes adjusted by performing machining such as grinding on the heat-treated compact.